The present invention relates to an elastic fluorohydrocarbon resin and a polymerization method for producing the same.
There are various fluorine-containing crystalline polymers which serve as synthetic resins and have wide application by reason of their characteristic properties attributed to the presence of C-F bond, such as good weather resistance, good chemical resistance and good heat resistance. However, these crystalline resins are unsuited to some uses wherein elasticity or flexibility of the employed resin is a matter of importance, as in the cases of pipes, gaskets, sealing elements, etc.
Where elasticity or flexibility is required besides the favorable properties of fluorohydrocarbon resins it is usual to use fluoroelastomer. However, fluoroelastomers fully exhibit their characteristic physical properties and particularly dynamic properties only after completion of a cross-linking process including the steps of kneading the fluoroelastomer in unvalcanized state with the addition of a cross-linking agent, fillers and stabilizers and of subjecting the kneaded rubber to a heat treatment to accomplish cross-linking. It is inevitable, therefore, that molding of fluoroelastomer requires more complicated operations than molding of crystalline fluorohydrocarbon resins, and often restrictions are placed on the shapes of the fluoroelastomer articles. Besides, fluoroelastomers after the cross-linking process can hardly be remelted for the purpose of additional processing or working.
To obtain a fluorine-containing resin which has a sufficient elasticity but does not need any cross-linking treatment, it has been tried to copolymerize a fluorine-containing monomer that is capable of providing a crystalline polymer having a relatively low glass transition temperature with a different monomer that is capable of sufficiently lowering the degree of crystallinity of the resultant copolymer. However, copolymers obtained by this method generally become lower in melting temperature and, hence, in the upper boundary of the temperature ranges in which the respective copolymers are practicable. Besides, the copolymers tend to undergo considerable changes in the modulus of elasticity with changes in temperature within the aforementioned ranges.
Also it has been tried to obtain a desirable resin by blending a crystalline fluorohydrocarbon resin with either a plasticizer or an elastic polymer. In practice, however, not so many kinds of plasticizers and polymers are known as sufficiently high in mutual solubility with crystalline fluorohydrocarbon polymers. Even when a blending material relatively high in the mutual solubility can be used, often it is impossible to use a desirably large amount of the blending material without adversely influencing the properties of the blended resin compositions. Furthermore, the blended resin compositions are liable to locally remain in a heterogeneously mixed state and, therefore, fail to fully exhibit the expected physical properties.
U.S. Pat. No. 4,472,557 corresponding to JP-A-58-206615 discloses an elastic fluorohydrocarbon resin and a method of producing the same. In this method, a fluorine-containing elastomeric polymer is graft-copolymerized with a fluorine-containing crystalline polymer in 1,1,2-trichlorotrifluoroethane. One of the elastomeric and crystalline polymers, that is, a trunk polymer, is prepared by copolymerizing at least one fluorine-containing monomer with at least one unsaturated monomer that has peroxy bond. The thus prepared elastomeric or crystalline polymer is washed and then dried in vacuum prior to the graft copolymerization.
JP-A-3-269008 discloses a modification of the method of U.S. Pat. No. 4,472,557. In this modification, the graft copolymerization is conducted in a solvent mixture of a good solvent capable of dissolving the fluorine-containing elastomeric polymer and a bad solvent not capable of dissolving the fluorine-containing elastomeric polymer. Examples of the good solvent are ketones, esters and chlorine-containing compounds such as 1,1,2-trichloro-1,2,2-trifluoroethane. Examples of the bad solvent are alcohols (e.g., t-butanol) and saturated hydrocarbons.
JP-A-1-292013 discloses another modification of the method of U.S. Pat. No. 4,472,557. In this modification, the graft copolymerization of a fluorine-containing elastomeric copolymer which is in the form of latex is conducted by redox polymerization in water.
It is generally known that an unsaturated peroxide alone is extremely unstable and thus liable to explode by heat, impact, and the like. However, if such monomer is dissolved in a solvent, it can be safely stored and handled with usual care. As solvents for dissolving unsaturated peroxides, inert solvents such as 1,1,2-trichlorotrifluoroethane and hydrocarbons such as toluene and isoparaffin are generally used. However, the use of 1,1,2-trichlorotrifluoroethane, causing the ozone-layer depletion, is greatly limited, and thus a demand for an alternative to 1,1,2-trichlorotrifluoroethane has been increasing. On the other hand, when the above-mentioned unsaturated peroxide dissolved in a hydrocarbon is used for producing the trunk polymer, the polymerization may terminate at an early stage due to the chain transfer reaction of the radicals of the polymerization initiator or of the monomer, to the solvent. As a consequence, the trunk polymer may become substantially lowered in yield or in molecular weight.
It is general to add various stabilizers to resins for providing the resins with thermal stability. However, if these stabilizers are added to fluororesins, this may cause the fluororesins to have stains or to become inferior in mechanical and chemical characteristics. It has been tried to add various stabilizers to an elastic fluorohydrocarbon resin prepared in accordance with the method of U.S. Pat. No. 4,472,557, for improving thermal stability of this resin, without damaging mechanical and chemical characteristics thereof. However, this could not lead a good result.